a) Field of the Invention
The present invention relates to a wide-angle zoom lens system which has a high vari-focal ratio, and is to be used with silver salt cameras, video cameras and so on.
b) Description of the Prior Art
There is conventionally known, as a preferable zoom lens system, the so-called negative-leading type zoom lens system which uses a lens unit having a negative refractive power as a first lens unit disposed therein. As examples of the negative-leading type zoom lens systems, there are known a zoom lens system which consists of a first lens unit having a negative refractive power, and a second lens unit having a positive refractive power, and other zoom lens systems each of which consists, as exemplified by a zoom lens system disclosed by Japanese Patent Kokai Publication No. Sho 58-95,315 or Kokai Publication No. Sho 58-217,910, of a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, a third lens unit having a positive refractive power and a fourth lens unit having a positive refractive power. Further, there are known still other zoom lens systems such as a lens system disclosed by Japanese Patent Kokai Publication No. Hei 2-259,614 which consists of a first lens unit having a negative refractive power, a second lens unit having a negative refractive power and a third lens unit having a positive refractive power, and a lens system disclosed by Japanese Patent Kokai Publication No. Hei 4-116,613 consisting of a first lens unit which has a negative refractive power and is kept fixed during a zooming operation, a second lens unit which has a negative refractive power and a function to correct a location of an image surface, a third lens unit which has a positive refractive power, and a fourth lens unit which has a negative refractive power and is kept fixed during a zooming operation, and a fifth lens unit which has a positive refractive power.
Out of the conventional examples described above, the zoom lens system consisting of two lens units has a vari-focal ratio which is as low as 2 or so and can hardly be enhanced.
Further, out of the conventional examples of the zoom lens systems each of which consists of four lens units, the zoom lens system disclosed by Japanese Patent Kokai Publication No. Sho 58-95,315 has a vari-focal ratio of approximately 4, but has a telephoto ratio as high as 1.8 at a tele position thereof and a large size, uses lens components in a number as large as 14 and requires a high manufacturing cost. Furthermore, the zoom lens system disclosed by Japanese Patent Kokai Publication No. Sho 58-217,910 has a vari-focal ratio of 3.5, but uses lens components in a number as large as 16, requires a high manufacturing cost and has a large size.
The zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei 2-25,916 uses three lens units, two of which are obtained by dividing, into two, the first lens unit having the negative refractive power which is disposed in the above-mentioned zoom lens system consisting of the two lens units. The zoom lens system disclosed by this Japanese Patent is configured so as to correct aberrations mutually with a first lens unit over an entire zooming range and a second lens unit, and correct aberrations in a third lens unit. Due to the composition of the zoom lens system described above, the third lens unit consists of a large number of lens components for a low vari-focal ratio of 2.5 and the second lens unit is not used effectively for enhancing the vari-focal ratio of the zoom lens system.
Moreover, the zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei 4-116,613 is configured so as to use five lens units by dividing, into two, the first lens unit of the zoom lens system consisting of the four lens units, and has a telephoto ratio of 6 or higher at a tele position thereof, a large size as a whole and a low vari-focal ratio of 3 due to a fact that the function to correct an image surface, which is allocated to the third lens unit in the conventional zoom lens system, is imparted to the second lens unit of this zoom lens system and a vari-focal function is not allocated to this second lens unit.